Effects Of Water Conditions And Humic Acids On The Adsorption And Transformation Of Cd In Paddy Soil

Soil heavy metal pollution is an increasingly serious environmental problem,and Cd pollution in paddy soil has attracted much attention because of the ability of rice to absorb Cd in the enriched soil.The threat of heavy metal Cd to the soil environment is not only related to the total amount,but also closely related to the proportion and transformation of heavy metal forms in the soil.Because humic acid is an important active component of soil organic matter and contains a variety of functional groups which can provide adsorption sites for heavy metals,it has become an important material for remediation of soil Cd pollution.Different water managements have important effects on the activity of Cd in soil.Therefore,batch experiments were conducted to investigate the effects of fulic acid(FA)and humic acid(HA),significant parts of humic acids,on the adsorption and desorption characteristics of Cd in soil.At the same time,the effects of different water conditions and humic acids on the speciation distribution of Cd in soil,the transformation in speciation and the conversion rate of exchangable Cd were studied by laboratory soil incubation experiment.The main results were as follows:1)The adsorption capacity of Cd increased with the increase of Cd concentration in paddy soil.The adsorption of Cd can be matched well with the Langmuir equation and Freundlich equation.FA can inhibit the adsorption and desorption of Cd in soil,while HA can promote the adsorption and desorption of Cd in soil,and the effect is positively correlated with the dosage.The adsorption kinetics of Cd in paddy soil can be divided into fast reaction stage and slow reaction stage.The quasi-first-order equation and quasisecond-order kinetic equation could be used to better describe the adsorption kinetics process,and the quasi-first-order kinetic equation had better fitting effect.Both HA and FA can improve the adsorption rate of Cd in paddy soil.2)After the soil incubation experiment,the exchangeable Cd accounted for the largest proportion among all the forms.Compared with the corresponding form distribution of CK,the changes of Cd in paddy soil treated with the same humic acid were the smallest at 75% field water capacity,followed by wet-dry rotation,and at water flooding,the exchangeable Cd(Exc-Cd),the organic matter bound Cd(OM-Cd)and the residual Cd(RES-Cd)distribution,compared with CK,changed more than those of the other two forms,while the distributions of carbonate bound Cd(CAR-Cd)and the iron manganese oxides bound Cd(IMOX-Cd)were smaller.Under the same water condition,the addition of FA increased EXC-Cd and OM-Cd,but reduced CAR-Cd,IMOX-Cd and RES-Cd in paddy soil.While the EXC-Cd,CAR-Cd,IMOX-Cd and RES-Cd were decreased and OM-Cd was increased with the addition of HA.Different water management methods and different humic acids treatments had different effects on the distribution variation of Cd forms in paddy soil.3)The incubation process of Cd in paddy soil was divided into a rapid response stage in the first 30 days and a slow response stage in the last 60 days.In the rapid reaction stage,the proportion of EXC-Cd in the soil decreased rapidly with the time prolonging,while the OM-Cd and RES-Cd increased rapidly with the time prolonging.The IMOXCd in the soil increased rapidly with time under 75% field capacity and wet-dry rotation,but increased slowly under water flooding.When the field capacity was 75%,CAR-Cd increased slightly in the first 15 days and decreased slightly in the second 15 days.It increased slightly in the first 10 days and decreased slightly in the second 20 days under wet-dry rotation.When under water flooding,it increased slightly with the prolonging of time.At the three water management methods,the proportions of all forms of Cd showed the most significant change was EXC-Cd,followed by OM-Cd,Res-Cd and IMOX-Cd,Car-Cd of that was the smallest.4)In the process of soil incubation,the addition of FA made EXC-Cd decrease more slowly,IMOX-Cd and RES-Cd increase more slowly,and OM-Cd increase more quickly.At 75% field capacity and wet-dry rotation,the addition of FA made CAR-Cd increase more slowly at first and then decrease more quickly,and at water flooding,it increases more slowly.The HA treatment reduced EXC-Cd more quickly,and the effects of it on the changes of the proportion of other Cd forms were basically the same as those of the FA treatment.Compared with the HA treatment,FA treatment had a stronger effect on the variation amplitude of Cd in EXC-Cd,OM-Cd and RES-Cd under 75% field water capacity.The change amplitudes of EXC-Cd,IMOX-Cd,OM-Cd and RES-Cd were more strongly affected by the HA treatment under wet-dry rotation.Under water flooding,the effects of FA treatment on the transformation of EXC-Cd IMOX-Cd and OM-Cd in soil were more obvious,but on that of the other two forms of Cd were relatively weak.5)In the soil incubation process,the rapid reaction stage of EXC-Cd can be better with an analytical solution of the diffusion equation ln Ce = A + BT fitting regression analysis.Under the condition of same humic acids treatment,the transformation rate of EXC-Cd in paddy soil generally presented that the fastest was under water flooding,the second was under dry-wet rotation,and the slowest was under 75% field capacity.The addition of FA decreased the conversion rate of EXC-Cd,while the addition of HA increased the conversion rate of EXC-Cd,and the effects were positively correlated with the amount of addition.6)FA can activate Cd in soil,while HA can passivate Cd in soil.Compared with the other two water conditions,Cd is more likely to reach a stable state under water flooding.